The present invention relates to a process for fabricating a fuel rod in which opposite ends of a fuel tube containing a plurality of fuel pellets are sealed with end plugs, and especially to an upper end plug used for said process.
In a conventional method for fabricating a fuel rod, a plurality of fuel pellets are inserted into an elongated fuel tube; a lower end plug is pressed into the lower opening of the fuel tube; a coiled-spring is inserted into the upper opening of the fuel tube; then an upper end plug having an aperture is pressed into the upper opening of the fuel tube. Then, the peripheral surfaces of the end plugs are welded to the fuel tube respectively; helium gas is injected into the fuel tube through the aperture; and the aperture is sealed by welding.
For sealing the aperture of the upper end plug in a conventional manner, an arc welding using a tungsten electrode is applied. However, after an arc is generated between the tungsten electrode and the periphery of the aperture, it is difficult to control the directionality of the arc in a limited area on the end face of the upper end plug, causing the arc to move freely on the end face of the upper end plug. As a result, in some cases, the arc causes melting of a shoulder portion of the upper end plug, because the arc often moves radially outwardly away from the center of the aperture, causing an improper melting of the shoulder portion. Therefore, in order to solve the problem such as above, the applicant suggested a method for welding the center of the aperture stably. In the method, an upper end plug is used having a protrusion formed around the periphery of the aperture of the upper end plug. The protrusion is employed as a lead portion for leading an arc generated in a welding (refer to Laid-open number 62-75377). On the other hand, in a conventional manner, the condition of the welding portion is inspected by transmitting X-ray through the welded portion, but it has been desired to change the X-ray inspection to an ultrasonic inspection. However, a chamfer is formed at the intersection between the end face and the outer peripheral surface of the upper end plug. The edge between the chamfer and the outer peripheral surface of the upper end plug is positioned in the inspection area of the ultrasonic beam. This disturbs the ultrasonic beam irradiated to the inspection area, and the stable inspection cannot be achieved. Therefore, it is difficult to irradiate stably the inspected area with ultrasonic beam. Although it is possible not to form the chamfer, the fabricating of the fuel rod and an assembling process for a fuel assembly may be difficult.